Before I had my bad ass 7800GTX SLI setup, I had an X800XT. The X850XT is faster- but not by a whole lot. Getting to the point, COD2 didn't run all that great at 1280x1024. It only dropped below 30fps a few times throughout the game, but it never "blew me away" in terms of performance either. I didn't use FSAA or 8x Anistropic filtering either.
My suggestion would be to not get an x800 based card if you want good performance in just COD2. COD2 was designed around the 6800/7800 series and not ATI's cards. Save an extra $50 and get a nexgen card like the 7800GT or even a 6800 series. Here are some benches (note, everything's maximum like AA and Anistropic filtering, so frame rates won't be THAT low in real life)
As you can see, COD2 benefits greatly from nVidia based cards. And since this one of the only games you play- it would make sense to go w/ nVidia in this case. All sports games run great on both cards (generally speaking, sports games are not demanding).